Why do I get the error "Camera not Multi-View Camera"?
When using the "Stereo 3D" mode; there only needs to be one camera present in the scene. The left and right cameras are created as children of the active camera, and their settings are controlled from the "Properties Window" -> "Object Data" tab; in the "Stereoscopy" panel.

Using "Multi-View" each camera needs to be added and positioned manually, however there can be more than two. The "Properties Window" -> "Object Data" tab "Stereoscopy" panel is not added in this mode.  

The "Properties Window" -> "Render Layers" tab, "Views" panel contains a list of camera "render views", with left and right already preset.
For each of these render views there is an associated "Suffix" text field.
In "Stereo 3D" mode this suffix field simply appends this text to the filename of the render of each camera.
In "Multi-View" mode this suffix field not only appends the text as above, it also identifies each camera producing a render view, therefore each camera needs to be renamed and should end with the matching suffix (case sensitive).  

The error; Camera "Camera_XXX" is not a multi-view camera, seems to occur when the name of the active camera does not end with one of the suffixes defined in the "render views" list, or if the any of the "render views" list suffixes are not assigned to a camera.
(This error message seems to return the name of the active camera, and not necessarily the name of a camera that is not assigned to a "render view".)